Soil contamination has become a major problem in the developed world. Chemicals from farms, underground storage units, industrial complexes and even residential areas are being released into the environment, contaminating surrounding soil, and threatening local water supplies. Particularly concerning is a group of chemicals known as VOCs. VOCs are volatile organic compounds found in products ranging from common household cleaners to gasoline additives. VOCs like trichloroethylene, benzene, toluene, and xylene have been linked to various environmental and health problems including cancer. The concern is that VOCs present in contaminated soil will seep into ground water.
The type of remediation used at a particular site is determined by a number of factors including, but not limited to: the diffusion constant of the contaminant, the porosity of the soil, the soil tortuosity, absorption/adsorption of the contaminant into/onto the soil, the ability of the adjacent soil to supply adequate thermal energy to vaporize the liquid contaminant, and other factors. See, U.S. Pat. No. 5,893,680, issued to Lowery et al., on Apr. 13, 1999, col. 2. Extremely contaminated sites often require costly remediation like excavation and/or thermal destruction, however, less contaminated sites can be managed with cheaper and less intrusive methods such as soil vapor extraction.
Soil vapor extraction (SVE) is a method that physically separates volatile contaminants from soil in vapor form. SVE is an in situ treatment method that uses vacuums, blowers and extraction wells to induce airflow in the subsurface to enhance the volatization of VOC compounds from unsaturated soil. Some vapor extraction methods also use injection wells to pump ambient air into the contaminated soil to help enhance the removal of volatile soil contaminants (this technique is known as air venting). Placing several injection wells around an extraction well creates a vacuum within the extraction well which assists in drawing the contaminants to the surface.
Following volatization, contaminants are drawn to the surface as a vapor by a vacuum where they can be treated and/or released into the atmosphere. In general, SVE is only effective in removing volatile compounds with a Henry's law constant of greater than 0.01 or a vapor pressure greater than 0.5 mm Hg (0.02 inches Hg).
There are two main types of soil vapor extraction methods: active soil vapor extraction (ASVE) and passive soil vapor extraction (PSVE). ASVE systems use active means such as pumps and/or vacuums to draw out soil contaminants while passive vapor techniques rely on less active forces such as changes in atmospheric pressure within and above the contaminated soil. Exemplary ASVE systems can be found in U.S. Pat. No. 4,730,673 issued to Payne; U.S. Pat. No. 5,160,217 issued to Metzer et al.; and U.S. Pat. No. 6,109,358 issued to Mc Phee et al. Payne is typical and teaches a closed loop system for removing volatile contaminants comprising: an extraction well surrounded by several injection wells. Pressurized air is actively injected into the soil through the injection wells and the air along with volatized chemicals are withdrawn under vacuum through the extraction well. The extracted air is subsequently processed by several scrubbers and filters and is then reinjected into the soil.
The disadvantages of using active vapor extraction include the high cost and complexity associated with powered pumps and vacuums. In addition, high volume extraction methods often exceed environmental air quality standards which in turn leads to costly air treatment.
Passive vapor pressure extraction (PSVE) is a soil remediation process that utilizes ambient meterological conditions in place of electrical or mechanical pumps and/or vacuums. An exemplary passive vapor extraction method can be found in U.S. Pat. No. 5,893,680 issued to Lowry et al. Lowry teaches utilizing naturally occurring barometric pressure oscillations between the air in the contaminated soil and the air pressure at the land surface to assist in the upward movement of volatile contaminants to the land surface. The obvious advantages of PSVE are the cost savings from not having to rely on powered pumps/vacuums and the fact that such systems are more easily implemented in remote locations.
The disadvantage of PSVE is that meteorological conditions that drive PSVE are often inconsistent in both time and degree. Thus PSVE tends to be less reliable and less effective than ASVE. Both ASVE and PSVE can be combined with other remediation techniques including: bioremediation (bioventing), air sparging, fracturing, chemical treatment and thermal enhancement to increase effectiveness.
The present invention teaches a system for enhanced passive soil vapor extraction (EPSVE) that combines the benefits of active and passive vapor extraction. The present systems provide a more consistent injection air source than is possible using traditional PSVE systems while eliminating the need for powered pumps and/or vacuums found in ASVE.
Furthermore, one embodiment of the present system contains volatile soil contaminants within an impermeable/semi-permeable barrier. The combination of vacuum forces and the use of a barrier allows containment of severely contaminated sites while more extensive remediation is planned and/or performed.